Add a string_view version of startswith

Add a string_view version of startswith

Makes sure that the string is longer than prefix, so that strncmp will
do the right thing even if the string is not null-terminated.

For use in my string_view conversion patch:
https://sourceware.org/ml/gdb-patches/2019-10/msg00030.html
https://gnutoolchain-gerrit.osci.io/r/c/binutils-gdb/+/125

gdb/ChangeLog:

2019-10-28  Christian Biesinger  <cbiesinger@google.com>

	* gdbsupport/common-utils.h (startswith): Add an overloaded version
	that takes gdb::string_view arguments.

Change-Id: I5389855de2fd70e7065a789a79374b0693651b71